PURPOSE: To attain broad band independently of the adoption of special structure by setting one resonance frequency of electroacoustic converters of transmission and reception systems to the upper limit frequency of the transmission band or above and setting the other to the upper limit frequency or below so as to eliminate the difference from the transmission band width depending on the variation in the characteristic of the electroacoustic transducers.

CONSTITUTION: The sensitivity of a sound wave transmitter 5 and a sound wave receiver 6 is as shown in figures A, B respectively, the sensitivity of the transmitter 5 has a slope of 12dB/oct, the resonance frequency fT has a peak in response to its Q, the resonance frequency fR of the receiver 6 has a peak in response to its Q and the sensitivity has a slope of -12dB/oct. The transmitter 5 and the receiver 6 are conventional ultrasonic wave transmission and reception devices, the resonance frequency fT is increased more than the upper limit frequency fU with respect to the band required for data transmission and the resonance frequency fR of the receiver 6 is decreased more than the lower limit frequency fL of the said band. Thus, the transmission band is widened by using a general electroacoustic transducer. Then both the electroacoustic transducers for the transmission system and the reception system are used at frequency other than the resonance point to attain the data transmission with high quality at a broad band and they withstand tight environment further.